Optical disks have become widely used in part due to their relatively high storage capacity. Whereas a 31/2-inch floppy disk stores merely 1.44 MB (megabytes) of data, a 12-cm compact optical disk can store upwards of 650 MB. The increased track density and the decreased pit size on a same size DVD-ROM provides 4.7 GB (gigabytes) of memory capacity. Optical disks are therefore increasingly becoming the most popular portable media for audio/video entertainment and data storage. Future development promises to bring increased memory capacity, such as double-sided, dual layer high definition (HD) DVD-ROM of 30 GB, and much shorter seek and access times to optical disk technology. Organizing optical disks in a jukebox that contains dozens to hundreds of disks and several disk drives can form very large archives, thus substantially increasing the value of optical disks as a versatile removable data storage media.
The market demand for optical disk drives is phenomenal. Audio CD players have become a necessary component in home entertainment appliances. The installed base of CD-ROM drives, now an estimated 195 million plus worldwide, will peak after the millennium. DVD, promising a new level of quality and convenience for movies, music, multimedia and interactive software, and digital analysis and storage, will inevitably revolutionize the way we view entertainment and gather information, and are estimated to replace the CD and dominate the optical disk market by the year 2002.
Increased availability of CD and DVD products, coupled with the availability of increasingly faster microprocessors, has created an enormous need for ever-faster optical disk drives. As a result, disk drives capable of operating at multiplied speeds of standard drives are becoming available. While the very first CD-ROM drive, introduced in 1991, operated at 1X speed, the performance of CD-ROM drives has leapfrogged from 8X to 24X speeds over the past year (1997), defying everything from overheating to increased vibration. The notion that faster is better has flattened the CD-ROM drive development cycle to approximately six months. The fastest drives available at present operate at 40X.
Currently available techniques for designing such high speed drives is limited to increasing the rotating speed of the optical disk to reduce data access latency and increase data transfer rate. Unfortunately, a main drawback of using constant angular velocity instead of constant linear velocity is that the data transfer rate across the entire disk is not uniform. Take, for instance, a CD-ROM specified as 24X by the manufacturer. While the data on the outside track may indeed be transferred at 24X, the rate on the inner tracks, where most of today's software is located, is only typically between 12X and 16X. It is unlikely that the manufacturers will be able to deliver reliable drives at or higher than 32X. The very high spindle speed compromises the performance reliability of such optical disk drives by creating additional cooling requirements and various stability issues.
To accelerate the data transfer rate further, another obvious alternative to increasing the disk angular velocity is by reading multiple data tracks simultaneously. Several patents have disclosed attempts to accomplish such a goal. U.S. Pat. No. 4,094,010 to R. Pepperl et al. describes a multi-channel optical disk storage system wherein a single beam is split into several read beams by using a series of partially transmitting beam splitters. In such a scheme, all the different optical elements must be aligned precisely relative to each other to achieve the highest packing density while preventing cross talk between the multiple beams or focused beam spots. Optical alignment is further complicated by thermal drifts, and the adjustment process can be tedious and time consuming. This need and the technical difficulties of achieving stringent optical alignment will inevitably slow down the data transfer rate, thereby defeating the purpose of using a multi-beam configuration.
U.S. Pat. No. 4,074,085 to J. T. Russell describes another multi-beam scheme, in which multiple illumination sources are applied to provide a plurality of record/read beams. The need for optical alignment as stated above imposes similar problems in this design. U.S. Pat. No 4,449,212 to C. W. Reno discloses an attempt to use multiple beams split from the output of a single laser to retrieve data. The beams are independently modulated by an acousto-optic device to record and playback data simultaneously. One skilled in the art will note that the extra device will inevitably complicate the servo system and increase the volume of the embodiment. U.S. Pat. No. 5,619,487 to T. Tanabe, et al. submits an ingenious proposal for two beams to read three tracks, but the sequential integration of the read information along the track is in practice difficult to accomplish.
Alternatively, U.S. Pat. No. 5,426,623 to A. Alon, et al. describes a broad beam illumination approach to multi-track reading. In this case, a static illumination/detection section provides a broad incoherent laser beam through a movable optical head section to illuminate several tracks at once. The reflected beam is directed onto a pixel array on the imaging detector. Unfortunately, incoherent light is very disadvantageous when used in CD-ROM pick-up systems. One of the main disadvantages of this broad beam design is that it requires high-energy output of the initial laser beam, risking thermal degradation of the optical disk surface and adding further cooling requirements. The incoherency of the light reduces image contrast provided by interference effect. The broad area of reflected disk image makes the algorithm for detector array pixel analysis extremely complicated and potentially inaccurate or unreliable.
In yet another design described in U.S. Pat. No. 5,729,512 to A. Alon, a multi-beam approach is proposed. A diffraction grating splits a single laser beam into seven evenly spaced and discrete beams to read data from seven adjacent tracks. The central beam is responsible for focusing and tracking. Seven separate optical pickups read the reflected beams and pass the signals through an integrated circuitry that multiplexes the data while performing focusing and tracking error calculations and corrections. Current commercially available multi-beam CD-ROM drives operate at 40X, offering a maximum data transfer rate of 6.0 MB/sec. Concomitantly, the ceiling of laser output energy destines the split beams at the border to be low, thus compromising the accuracy and reliability of the reading and complicating the signal analysis system. The same problems also limit maximum number of split beams (currently at seven) for reading even greater number of tracks at the same time. Additional optical elements required by the device further compound the bulkiness of the pick-up system and increase the manufacturing cost.
Implementing simultaneous multi-track reading capability in optical disk drives presents further difficulties related to focusing and tracking operations. Two conventional methods for focusing and tracking in CD-ROM drives are described on pages 338-348 of L. Buddine and E. Yong, The Brady Guide to CD-ROM, (1987), published by Brady, New York, which is incorporated herein by reference. A typical system by Philips uses a single beam and swing arm assembly with push-pull tracking and Foucault focusing, while a system by Sony applies a triple-beam and sled combination with screw tracking and astigmatic focusing. Both systems have immobilized and separate illumination and detection sections and complicated optics for directing the laser beam(s) to the optical disk and re-channeling the reflected light to the detectors at a different location than the laser(s).
Direct adaptation of either of these two tracking and focusing methods and apparatus in multi-track reading systems would require additional, costly optical components and risk compromises in accuracy and reliability of data retrieval. Patents regarding such modifications can be found in the prior art. U.S. Pat. No. 5,708,634 to A. Alon, et al. describes a modification of the Sony focusing system using quadrant detectors for application in the aforementioned design of broad beam illuminating multiple optical tracks. Also, U.S. Pat. No. 5,728,512 to A. Alon describes an error compensation system for tracking and magnification using servo systems to analyze the signals from photodiodes with multiple divisions.
Increased complexity in these data reading and focusing and tracking error correction systems is likely to increase the volume of the embodiment and the adjustment response latency in a manner that is counterproductive to the twin goals of achieving compactness and high data transfer rate. However, the inventors have realized that recent advances in microlasers and micro-scale photo detectors and molded aspheric optics provide options of co-localizing and consolidating multiple elements thereby making better alternative designs with more significant advantages possible.
Because of size constraints, conventional optical disk reader heads only use a single edge-emitting laser and one or more detectors. The lower size limit for fabrication of edge-emitting lasers is 50 microns in width and 100-200 microns in length. In comparison, Vertical Cavity Surface-Emitting Lasers (VCSELs) can be made to minute circular units of less than 10 microns in diameter. Advantageously, VCSELs' vertical beam emission property allows them to form a two-dimensional laser array. Current VCSELs emit at wavelengths between 650 nm and 980 nm. They are made of GaAs/GaAlAs, InGaAs, GaInP and AlGaAs, InP, having a sandwiched structure of multi-layer reflective surfaces between which multiple quantum-wells are formed. Two-dimensional laser array can be manufactured through Molecular Beam Epitaxy (MBE) or Metal Organic Chemical Vapor Deposition (MOCVD) and lithography.
U.S. Pat. No. 5,526,182 to J. L. Jewell et al. describes the implementation of 1-dimensional and 2-dimensional arrays of VCSELs in optical memory system with multi-beam reading multi-track capacity. The present inventors have discovered that a main drawback of this design is that the detectors are not incorporated with the micro-lasers on the same array block. A preferable choice of detector for VCSEL is Resonance Cavity Photo Diode (RCPD) because of similar manufacturing process and RCPD's high sensitivity (&gt;85% of quantum transfer efficiency) in a narrow spectral region around VCSEL's wavelengths.
The inventors have further explored the integration of lasers and detectors on the same chip. Ortiz et al. have first successfully demonstrated in 1996 (Electronics Letters, Vol. 32, No. 13, pp. 1205-6, which is incorporated herein by reference) the monolithic integration of VCSELs and RCPDs on the same substrate with the same epilayer design. Construction of VCSEL and RCPD arrays with control circuits and signal amplifiers on the same movable block allows the functional integration of all the optical interconnection and optoelectronic interface functions in a very compact format. It also simplified the optical interconnect package by facilitating the integration of optoelectronic components with beam tracking, focussing, and magnification functions. The application of molded aspheric lenses will also markedly simplify the optical system by packing more functions into a small element. Advanced semiconductor techniques such as different types of lithography have made it possible to fabricate microlasers and detectors on a massive scale at extremely low cost. The miniaturized pick-up system can be further modified into reading optical data encoded on wallet-size personal information and medical history cards such as those described in U.S. Pat. No. 4,745,268 to J. Drexler.
In view of the foregoing, it would be desirable to provide compact, fast, and low cost optical reading and servo control methods and apparatus for single-track reading of an optical disk.
It would also be desirable to provide compact, fast, and low cost optical reading methods and apparatus for simultaneous multi-track reading of an optical disk to achieve very high data transfer rates.
It would also be desirable to provide compatible tracking, focusing, and magnification methods and apparatus to be employed in the optical reading systems for single-track and multi-track optical data reading.
It would also be desirable to provide optical reading methods and apparatus with expandable capacities in the illumination and the detection sections to increase further the number of parallel-read tracks.
It would also be desirable to provide synchronized mobility and position adjustability to the illumination and the detection sections of the optical reading methods and apparatus for single-track and multi-track reading.
It would be still more desirable to provide optical reading methods and apparatus for single-track and multi-track reading using a single simplified and multi-functional optical element in place of multiple, complex, and expensive optical components.